Electrophotographic printers having a laser as the light source generally include an optical device which comprises the laser light source, laser beam shaping means and a beam scanning assembly. In recent years, such printers have been required to have a dot density (DPI: dots/inch) in the range of 100 to 1000, whereas different dot densities need different optical devices, consequently necessitating different printers.
Accordingly, Unexamined Japanese Patent Application No. SHO 59-117372 proposes a printer which is adapted to selectively give one of a plurality of different dot densities by automatically collectively controlling the laser beam diameter, laser modulation frequency, speed of rotation of a polygonal mirror for scanning with the beam and speed of rotation of the photosensitive drum.
However, since the prior-art printer has a single optical device which has a variable beam diameter, laser modulation frequency and rotational speed of the polygonal mirror, the device is complex in construction and becomes large-sized. Moreover, there is a limitation to the speed of the motor in varying the speed of the polygonal mirror, and a higher speed results in impaired durability and a lower speed involves uneven rotation. The use of one optical device thus imposes limitations on the range of dot density variations, so that there arises a need to prepare different printers for widely varying dot densities.